Avoidance of discontinuities when switching between modulation schemes

ABSTRACT

Modulator system ( 1 ) comprising modulators ( 2, 3, 4 ) for modulating input signals (A) according to different modulation schemes (8PSK, GMSK) cause discontinuities in the output signals (F) when switching between the schemes. By providing the modulator systems ( 1 ) with compensators ( 13, 22 - 26 ) for compensating amplitudes/phases of the output signals (F) of the modulator system ( 1 ) for discontinuities, these discontinuities resulting from modulation scheme changes are reduced to a large extent. This may be done before/after the pulse shapers ( 11, 21 ). The compensators ( 13, 22 - 26 ) comprise multipliers for multiplying pulse shaped modulated signals with complex valued waveforms (E), or for multiplying modulated signals with waveforms (S, T), or for multiplying complex valued signals (B, C, D) with complex valued phase offsets (X, Y, Z), which complex valued signals (B, C, D) are to be multiplied with mapped input signals. As a result, the output signals (F) and/or power amplifiers ( 33 ) situated after the modulator system ( 1 ) no longer need to be ramped down.

The invention relates to a modulator system comprising a first modulatorfor modulating an input signal according to a first modulation schemeand a second modulator for modulating the input signal according to asecond modulation scheme, to a transmitter comprising such a modulatorsystem, to a modulator, to a method and to a processor program product.

Examples of such a transmitter are mobile radio terminals and basestations and network nodes operating in a Global System for Mobiletelecommunication (GSM) supporting an Enhanced General Packet RadioService (EGPRS) or a Universal Mobile Telecommunication System (UMTS).

A prior art modulator system is known from the article “Combined GMSKand 8PSK Modulator for GSM and EDGE, by Peter Bode and Alexander Lampe,Philips Semiconductors Nuremberg, Germany, and Markus Helfenstein,Philips Semiconductor Zürich, Switserland, which discloses in its FIG. 5a first modulator (modulation scheme: eight Phase Shift Keying or 8PSK)and in its FIG. 6 a second modulator (modulation scheme: GaussianMinimum Shift Keying or GMSK). During a modulation scheme change,discontinuities in the output signal of the modulator system may arise,which increase the adjacent channel interference. To avoid this adjacentchannel interference, a power amplifier for amplifying this outputsignal is ramped down during the modulation scheme change. This rampingdown is realized by creating a zero output signal of the modulatorsystem through signal shaping during the modulation scheme change.

The known modulator system is disadvantageous, inter alia, due tocausing the power amplifier to be ramped down. This limits the possiblepower amplifiers to be used. Some power amplifiers may not be rampeddown and require an always non-zero signal at their input.

It is an object of the invention, inter alia, to provide a modulatorsystem having relatively little discontinuities in its output signalduring a modulation scheme change, without this output signal beingramped down.

Furthers objects of the invention are, inter alia, to provide atransmitter comprising such a modulator system having relatively littlediscontinuities in its output signal during a modulation scheme change,without this output signal being ramped down, and a modulator, a methodand a processor program product all having relatively littlediscontinuities in their output signal during a modulation schemechange, without this output signal being ramped down.

The modulator system according to the invention comprises a firstmodulator for modulating an input signal according to a first modulationscheme and a second modulator for modulating the input signal accordingto a second modulation scheme, which modulator system comprises acompensator for combining at least one modulator signal with at leastone waveform for compensating at least one signal parameter of an outputsignal for discontinuities resulting from a modulation scheme change.

By providing the modulator system with the compensator for compensatingone or more signal parameters of the output signal of the modulatorsystem for discontinuities, these discontinuities resulting frommodulation scheme changes are reduced to a relatively large extent. Thecombining like for example multiplying and/or adding of a for examplemodulated signal with a waveform allows at least one modulated signal tobe adapted in such a way that a discontinuity between a first modulationsignal modulated according to a first modulation scheme and a secondmodulation signal modulated according to a second modulation scheme issmoothened to a relatively large extent. Usually this will be doneduring a so-called guard interval located between some data symbols and(three) tail symbols respectively on one side and (three) tail symbolsand some data symbols respectively on the other side. Alternative guardintervals can be located between some data symbols on one side and somedata symbols on the other side.

A first embodiment of the modulator system according to the invention isdefined by further comprising at least one pulse shaper, with thecompensator being located after the pulse shaper. By locating thecompensator after the one or more pulse shapers of the modulators, thesemodulators themselves do not need to be amended.

A second embodiment of the modulator system according to the inventionis defined by the compensator comprising a multiplier for multiplyingthe modulator signal in the form of at least one pulse shaped modulatedsignal with the waveform in the form of a complex valued waveform, withthe at least one signal parameter comprising an amplitude and a phase.By providing the compensator when located after the one or more pulseshapers with the multiplier, discontinuities in the amplitude as well asin the phase of at least one pulse shaped modulated signal aresmoothened via one multiplier receiving the complex valued waveform.

A third embodiment of the modulator system according to the invention isdefined by further comprising at least one pulse shaper, with thecompensator being located before the pulse shaper. By locating thecompensator before the one or more pulse shapers of the modulators, thecompensator can be integrated into the modulators. Usually, a first partof the compensator will be integrated into the first modulator, and asecond part of the compensator will be integrated into the secondmodulator.

A fourth embodiment of the modulator system according to the inventionis defined by the compensator comprising at least one multiplier formultiplying the modulator signal in the form of at least one modulatedsignal with the waveform, with the at least one signal parametercomprising an amplitude. By providing the compensator when locatedbefore the one or more pulse shapers with at least one but usually twomultipliers, discontinuities in the amplitude of at least one modulatedsignal are smoothened via the one or two multipliers receiving thewaveform. This waveform may be in the form of a complex valued waveform,which possibly but not exclusively comprises a real valued waveformonly.

A fifth embodiment of the modulator system according to the invention isdefined by each modulator comprising at least one multiplier formultiplying a mapped input signal with a complex valued signal, with thecompensator comprising at least one multiplier for multiplying themodulator signal in the form of the complex valued signal with thewaveform in the form of a complex valued phase offset, with the at leastone signal parameter comprising a phase. By providing the compensatorwhen located before the one or more pulse shapers with the at least onebut usually three additional multipliers, one additional multiplier forone prior art multiplier in the first modulator and two multipliers fortwo prior art multipliers in the second modulator, thereby suggestingthat the second modulator is based on a two branch modulation scheme,discontinuities in the phase of at least one modulated signal aresmoothened. Thereto, per modulation scheme, usually a different phaseoffset will need to be multiplied to the complex valued signal.

A sixth embodiment of the modulator system according to the invention isdefined by the first modulation scheme being a Phase Shift Keyingmodulation scheme and the second modulation scheme being a GaussianMinimum Shift Keying modulation scheme.

The transmitter according to the invention comprises a modulator systemcomprising a first modulator for modulating an input signal according toa first modulation scheme and a second modulator for modulating theinput signal according to a second modulation scheme, which modulatorsystem comprises a compensator for combining at least one modulatorsignal with at least one waveform for compensating at least one signalparameter of an output signal for discontinuities resulting from amodulation scheme change, which transmitter further comprises a poweramplifier for amplifying the output signal. Of course, this outputsignal may be the output signal originating from the modulator system ora derived version thereof, like for example a digitized version thereof.

The modulator according to the invention for modulating an input signalaccording to a modulation scheme comprises a compensator for combiningat least one modulator signal with at least one waveform forcompensating at least one signal parameter of an output signal fordiscontinuities resulting from a modulation scheme change. Thismodulator either has the ability to adapt its modulation scheme, or isused in combination with an other modulator.

The method according to the invention for modulating an input signalaccording to a first modulation scheme and for modulating the inputsignal according to a second modulation scheme comprises a step ofcombining at least one modulator signal with at least one waveform forcompensating at least one signal parameter of an output signal fordiscontinuities resulting from a modulation scheme change.

The processor program product according to the invention for modulatingan input signal according to a first modulation scheme and formodulating the input signal according to a second modulation schemecomprises a function of combining at least one modulator signal with atleast one waveform for compensating at least one signal parameter of anoutput signal for discontinuities resulting from a modulation schemechange.

Embodiments of the transmitter according to the invention and of themodulator according to the invention and of the method according to theinvention and of the processor program product according to theinvention correspond with the embodiments of the modulator systemaccording to the invention.

The invention is based upon an insight, inter alia, that the rampingdown of an output signal of the modulator system is to be avoided, andis based upon a basic idea, inter alia, that discontinuities inamplitudes and/or phases of a signal can be smoothened by adapting atleast the last part of this signal before the discontinuity and/or atleast the first part of this signal after the discontinuity.

The invention solves the problem, inter alia, to provide a modulatorsystem having relatively little discontinuities in its output signalduring a modulation scheme change, without this output signal beingramped down, and is advantageous, inter alia, in that this modulatorsystem can be combined even with a power amplifier which may not beramped down and which requires an always non-zero signal at its input.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments(s) described hereinafter.

In the drawings:

FIG. 1 shows in block diagram form a modulator system according to theinvention comprising a compensator of a first type;

FIG. 2 shows in block diagram form a modulator system according to theinvention comprising a compensator of a second and a third type;

FIG. 3 shows in an upper graph (prior art) an absolute value of anoutput signal created without compensation and in a lower graph(invention) an absolute value of an output signal created throughcompensation before pulse shaping; and

FIG. 4 shows in block diagram form a transmitter according to theinvention.

The modulator system 1 according to the invention as shown in FIG. 1comprises a first eight Phase Shift Keying (8PSK) modulator 2 and asecond Gaussian Minimum Shift Keying (GMSK) modulator 3, 4 having afirst modulating part 3 (first branch) and a second modulating part 4(second branch). An input signal A is supplied to an input of a firstmultiplexer 5 and via a first output of the first multiplexer 5 suppliedto an input of the modulator 2 and via a second output of the firstmultiplexer 5 supplied to inputs of the modulating parts 3, 4. Inmodulator 2, the input signal A is supplied to aserial-to-parallel-converter 6 and then mapped by a mapper 7, afterwhich the mapped signal is multiplied through a multiplier 8 with asignal B and a resulting modulated signal is supplied to a first inputof a second multiplexer 9. An output signal of the second multiplexer 9is supplied to an upsampler 10 and then pulse shaped by a pulse shaper11 (a Finite Impulse Response (FIR) Filter), after which a first pulseshaped signal is added via an adder 12 to a second pulse shaped signalcoming from the modulating part 4.

In modulating part 3, the input signal A is supplied to a mapper 14,after which the mapped signal is multiplied through a multiplier 15 witha signal C and a resulting modulated signal is supplied to a secondinput of the second multiplexer 9. In modulating part 4, the inputsignal A is supplied to a Finite State Machine (FSM) 16 and then mappedby a mapper 17, after which the mapped signal is multiplied through amultiplier 18 with a signal D and a resulting modulated signal issupplied to a first input of a third multiplexer 19. An output signal ofthe third multiplexer 19 is supplied to an upsampler 20 and then pulseshaped by a pulse shaper 21 (a Finite Impulse Response (FIR) Filter),after which this second pulse shaped signal is supplied to the adder 12discussed before. The added first and second pulse shaped signals arecombined with a complex valued waveform E for compensating at least onesignal parameter like an amplitude or a phase of the output signal F fordiscontinuities resulting from a modulation scheme change, as discussedbelow.

The first modulator 2 modulates the input signal A according to a firstmodulation scheme 8PSK. Thereto, the first multiplexer 5 is controlledin such a way that the input signal is supplied to the modulator 2 andnot to the modulators 3, 4. The input signal A is serial-to-parallelconverted and then mapped and then multiplied with the signal B, withthe signal B for example being equal to exp j(3πk/8). The secondmultiplexer 9 is controlled in such a way that the resulting modulatedsignal is upsampled and pulse shaped. In a prior art situation, with themultiplier 13 not being present, the upsampled and pulse shapedmodulated signal forms the output signal F of the modulator system 1 incase of this modulator system 1 being in a 8PSK mode. Thereto, the thirdmultiplexer 19 is controlled in such a way that a second input of thisthird multiplexer 19 receiving a zero signal is coupled to the thirdmultiplexer's output.

The second modulator 3, 4 modulates the input signal A according to asecond modulation scheme GMSK. Thereto, the first multiplexer 5 iscontrolled in such a way that the input signal is supplied to themodulator 3, 4 and not to the modulator 2. In the modulating part 3, theinput signal A is mapped and then multiplied with the signal C, with thesignal C for example being equal to exp j(πk/2). In the modulating part4, the input signal A is processed by the FSM 16 and is mapped and thenmultiplied with the signal D, with the signal D for example being equalto exp j(π[k−1]/2). The second multiplexer 9 and the third multiplexer19 are controlled in such a way that the resulting modulated signals canbe upsampled and pulse shaped. In a prior art situation, with themultiplier 13 not being present, the upsampled and pulse shapedmodulated signals form the output signal F of the modulator system 1.

So, by controlling the multiplexers 5, 9 and 19, the modulation schemecan be selected. In case of a modulation scheme change, discontinuitiesin the output signal F of the modulator system 1 may arise, whichincrease the adjacent channel interference. To avoid increased adjacentchannel interference, these discontinuities can be avoided byintroducing multiplier 13 (a compensator of a first type). For example,in case of four GMSK symbols arriving from the adder 12 and being fedinto the multiplier 13 and then four 8PSK symbols arriving from theadder 12 and being fed into the multiplier 13 during a guard intervalcomprising eight symbols, the amplitudes for example equal 1, 1, 1, 1,0.7, 0.7, 0.7 and 0.7. To avoid amplitude discontinuities, theamplitudes of eight samples of the complex valued waveform E could bechosen as 1, 0.96, 0.91, 0.86, 1.14, 1.09, 1.03 and 1. The amplitudes ofthe output signal of the multiplier 13 then equal 1, 0.96, 0.91, 0.86,0.8, 0.76, 0.72 and 0.7. As a result, the amplitude smoothly decreasesfrom 1 to 0.7, and any amplitude discontinuities in the output signal Fhave been avoided. The same way, any phase discontinuities in the outputsignal F can be avoided by supplying the eight samples of the complexvalued waveform having appropriately chosen phases.

The modulator system 1 according to the invention as shown in FIG. 2corresponds with the modulator system 1 according to the invention asshown in FIG. 1, apart from the following. Instead of multiplier 13, nowmultipliers 25, 26 respectively have been introduced (a compensator of asecond type) located between multiplexers 9, 19 and upsamplers 10, 20respectively for multiplying the output signals of the multiplexers 9,19 with waveforms S, T respectively, and multipliers 22, 23, 24respectively have been introduced (a compensator of a third type),coupled to multipliers 8, 15, 18 respectively for multiplying thesignals B, C, D respectively with the complex valued phase offsets X, Y,Z respectively.

A possible setting of the multiplexers 9 (mux 9) and 19 (mux 19) duringa transition from GMSK to 8PSK is as follows:

Mux 9 . . . G G G G G G G G G G P P P P P P P P P P . . . Mux 19 . . . GG G G G G G G G G 0 0 0 0 0 0 0 0 0 0 . . .

The first column with symbols shows the last three data symbols, thesecond column shows three tail symbols, the third column shows eightguard symbols, the fourth column shows three tail symbols, and the fifthcolumn shows the first three data symbols.

The complex valued phase offset X for example equals exp j[φ_(8PSK)]=expj[arg(g[k])+πk/2+Φ_(GMSK)−arg(p[k+1])−3π(k+1)/8+ΔΦ], thereby suggestingthat the last GMSK symbols and the first 8PSK symbols are fed into themultiplexers 9, 19 at symbol interval k and k+1 respectively, and thatφ_(GMSK) denotes the phase offset of a preceding GMSK modulated burstand that A, denotes a phase angle defining the phase difference betweenthe last GMSK symbol and the first 8PSK symbol. In case all symbolsG[k], P[k] equal “1” in the guard interval, an appropriate choice for Δφis for example 3π/8. With this choice, the GMSK symbol fed into themultiplexer in symbol interval k looks like a preceding 8PSK “1” for the8PSK symbol passed to the multiplexer in symbol interval k+1. The phaseoffset φ8PSK is updated when switching from GMSK to 8PSK and remainsconstant during an 8PSK modulated burst.

A possible setting of the multiplexers 9 (mux 9) and 19 (mux 19) duringa transition from 8PSK to GMSK is as follows:

Mux 9 . . . P P P P P P P P P P G G G G G G G G G G . . . Mux 19 . . . 00 0 0 0 0 0 0 0 0 G G G G G G G G G G . . .

The first column with symbols shows the last three data symbols, thesecond column shows three tail symbols, the third column shows eightguard symbols, the fourth column shows three tail symbols, and the fifthcolumn shows the first three data symbols.

The complex valued phase offset Y, Z for example equals exp j[φGMSK]=expj[arg(p[k])+3πk/8+φ_(8PSK)−arg(g[k+1])−π(k+1)/2+Δφ], thereby suggestingthat the last 8PSK symbols and the first GMSK symbols are fed into themultiplexers 9, 19 at symbol interval k and k+1 respectively, and thatφ_(8PSK) denotes the phase offset of a preceding 8PSK modulated burstand that Δφ denotes a phase angle defining the phase difference betweenthe last 8PSK symbol and the first GMSK symbol. In case all symbolsP[k], G[k] equal “1” in the guard interval, an appropriate choice for Δφis for example π/2. With this choice, the 8PSK symbol fed into themultiplexer in symbol interval k looks like a preceding GMSK “1” for theGMSK symbol passed to the multiplexer in symbol interval k+1. The phaseoffset φ_(GMSK) is updated when switching from 8PSK to GMSK and remainsconstant during a GMSK modulated burst.

FIG. 3 shows in an upper graph (prior art) an absolute value of anoutput signal F after digital-to-analog-conversion created withoutcompensation and in a lower graph (invention) an absolute value of anoutput signal F after digital-to-analog-conversion created throughcompensation by multiplying the modulator signal in the form of thecomplex valued signal B, C, D with the waveform X, Y, Z in the form of acomplex valued phase offset X, Y, Z, before pulse shaping. Themodulation scheme change from 8PSK to GMSK takes place between symbolinterval 56 and 57, with all symbols P(k), G(k) chosen to have the value“1” in the guard interval (53-60).

When demanding equal peak values for GMSK and 8PSK, the samples of thewaveforms S, T should be “1” for 8PSK and about “1.5” for GMSK, whendemanding equal root mean square values these samples should all be “1”.

The modulator system 1 as shown in FIG. 2 calculates a phase offset Y, Zand rotates GMSK pulse shaping filter input symbols therewith, andcalculates a phase offset X and rotates 8PSK pulse shaping filter inputsymbols therewith, and calculates an amplitude waveform S, T andmultiplies GMSK/8PSK pulse shaping filter input symbols therewith.

The modulator system 1 as shown in FIGS. 1 and 2 minimizes adjacentchannel interference, improves the stability of phase and/or amplitudeloops controlled by the output signal F, does not require ramping andramping calculations, is simple, allows a fast transition betweenmodulation schemes, and offers amplitude smoothing when changing betweenequal root mean square values and peak values for GMSK and 8PSK forfree.

The transmitter 30 as shown in FIG. 4 comprises an input stage 31 forgenerating the input signal A for the modulator system 1, and comprisesfor example a digital-to-analog-converter 32 for converting the outputsignal F into an analog signal, and comprises for example a poweramplifier 33 for amplifying the analog output signal.

Alternative modulation schemes like for example 4PSK or 16PSK instead of8PSK and alternative modulator system constructions are possible.Instead of multiplying exp j(a) and exp j(b), the adding of a+b could beperformed, and vice versa: exp j(a)*exp j(b)=exp j(a+b).

A complex valued waveform comprises a real valued waveform, an imaginaryvalued waveform, or a combination of both.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.Use of the verb “to comprise” and its conjugations does not exclude thepresence of elements or steps other than those stated in a claim. Thearticle “a” or “an” preceding an element does not exclude the presenceof a plurality of such elements. The invention may be implemented bymeans of hardware comprising several distinct elements, and by means ofa suitably programmed computer. The mere fact that certain measures arerecited in mutually different dependent claims does not indicate that acombination of these measures cannot be used to advantage.

1. A modulator system comprising: a first modulator configured tomodulate an input signal according to a first modulation scheme; asecond modulator configured to modulate the input signal according to asecond modulation scheme; a compensator configured to combine at leastone modulator signal with at least one waveform to compensate at leastone signal parameter of an output signal for discontinuities resultingfrom a modulation scheme change, wherein each modulator comprises atleast one multiplier configured to multiply a mapped input signal with acomplex valued signal and the compensator comprises at least onemultiplier configured to multiply the at least one modulator signal inthe form of the complex valued signal with the waveform in a form of acomplex valued phase offset with the at least one signal parametercomprising a phase; and at least one pulse shaper coupled to an outputof the compensator.
 2. The modulator system of claim 1, furthercomprising: a least one upsampler coupled between the compensator andthe at least one pulse shaper.
 3. The modulator system of claim 2wherein the at least one signal parameter comprises an amplitude and aphase.
 4. The modulator system of claim 1 wherein the first modulationscheme is a Phase Shift Keying modulating scheme and the secondmodulation scheme is a Gaussian Minimum Shift Keying modulation scheme.5. The modulator of claim 1, further comprising an adder configured toadd the pulse-shaped output of the compensator to another pulse-shapedcompensated signal.
 6. A transmitter comprising: a first modulatorconfigured to modulate an input signal according to a first modulationscheme; a second modulator configured to modulate the input signalaccording to a second modulation scheme; and a compensator configured tocombine at least one modulator signal with at least one waveform tocompensate at least one signal parameter of an output signal fordiscontinuities resulting from a modulation scheme change, wherein eachmodulator comprises at least one multiplier configured to multiply amapped input signal with a complex valued signal and the compensatorcomprises at least one multiplier configured to multiply the at leastone modulator signal in the form of the complex valued signal with thewaveform in the form of a complex valued phase offset with the at leastone signal parameter comprising a phase; at least one pulse shapercoupled to an output of the compensator; and configured to amplify anoutput of the transmitter.
 7. A modulator to modulate an input signalaccording to a modulation scheme, the modulator comprising: acompensator configured to combine at least one modulator signal with atleast one waveform to compensate at least one signal parameter of anoutput signal for discontinuities resulting from a modulation schemechange, wherein the modulator comprises at least one multiplierconfigured to multiply a mapped input signal with a complex valuedsignal and the compensator comprises at least one multiplier configuredto multiply the at least one modulator signal in the form of the complexvalued signal with the waveform in the form of a complex valued phaseoffset with the at least one signal parameter comprising a phase; and atleast one pulse shaper coupled to an output of the compensator.
 8. Amethod of modulating an input signal according to a first modulationscheme and modulating the input signal according to a second modulationscheme, the method comprising: combining at least one modulator signalwith at least one waveform for compensating at least one signalparameter of an output signal for discontinuities resulting from amodulation scheme change, wherein the at least one modulator signalcomprises a mapped input signal multiplied by a complex valued signaland the combining comprises multiplying the at least one modulatorsignal in the form of the complex valued signal with the waveform in aform of a complex valued phase offset with the at least one signalparameter comprising a phase; and pulse-shaping the combined at leastone modulator signal and at least one waveform.
 9. The method of claim 8wherein the at least one signal parameter comprises a signal parameterhaving an amplitude.
 10. A processor program product for causing acomputer to perform a method, the method comprising: modulating an inputsignal according to a first modulation scheme; modulating the inputsignal according to a second modulation scheme; combining at least onemodulator signal with at least one waveform for compensating at leastone signal parameter of an output signal for discontinuities resultingfrom a modulation scheme change, wherein the at least one modulatorsignal comprises a mapped input signal multiplied by a complex valuedsignal and the combining comprises multiplying the at least onemodulator signal in the form of the complex valued signal with thewaveform in a form of a complex valued phase offset with the at leastone signal parameter comprising a phase; and pulse-shaping the combinedat least one modulator signal and at least one waveform.
 11. Theprocessor product of claim 10 wherein the at least one signal parametercomprises a signal parameter having an amplitude.